I know it's an older thread - but I got my 18.5mm yesterday. I noticed some interesting properties. In A mode, setting the aperture does make a click (by no means loud!) from F1.8 to about F4.5 and you can see the blades closing. After F4.5 it opens up again and only closes when you take the picture - I'm assuming to do with the autofocus needing a certain minimum aperture.

But in video mode, manual A, when you change the aperture, it's silent when the blades move.